If Jan Koum and Brian Acton hadn’t been turned down for jobs at Facebook, the lives of a billion or so people around the world might look somewhat different today. Their failure to get hired, however, left the two former Yahoo! employees with enough time on their hands to play around with an idea. And eight years ago, that idea became WhatsApp.

Like most incredibly lucrative inventions, it doesn’t sound like much; just a free, quick and easy mobile phone messaging service, allowing users to set up specific groups of friends around whom messages will be sent en masse. But last year it overtook traditional SMS text messaging in popularity and increasingly it’s weaving itself into the fabric of modern life, for what it really does is create private meeting places in a very public online world. In that sense, WhatsApp is beginning to turn friendship back into what it used to be before Facebook (which inevitably bought the app three years ago); not vast, sprawling networks of people you barely know but small, intimate circles of trust where like-minded people can share stuff that matters to them.

For political activists inside repressive regimes, WhatsApp groups can be a lifesaver

Sometimes it’s things that would be boring to anyone outside the circle, as with the legions of family WhatsApps used to share baby pictures, in-jokes and gently nagging messages from mothers to far-flung offspring at university. For teenagers, they’re places to dissect last Saturday night in excruciatingly minute detail, and develop their own intricate etiquette along the way. (It’s rude to ignore an unfolding group chat, since the app can let the rest of the group know who’s online and if they’ve read a post; but it’s just as rude to bombard the group with endless witterings or prolong the conversation after everyone else clearly wants to stop. The ethics of sneaking off with one member for a private chat behind the group’s back, meanwhile, remain a minefield.) But sometimes what’s shared is anything but dull.

Shortly after June’s general election, Tory MPs used WhatsApp groups to canvass backbench opinion about Theresa May’s prospects – so much more discreet than huddling in the corners of Commons tearooms, as plotters did in a more analogue age. They’re routinely used on all sides of the house to swap gossip, agree lines to take across groups of sympathetic MPs – Brexiters, say, or Labour moderates despairing of Jeremy Corbyn – and support individuals under pressure. They’ve played a pivotal part in exposing sexual harassment in both politics and journalism, with victims swapping names via a “whisper network” of like-minded WhatsAppers. And for political activists inside repressive regimes, they can be a lifesaver.

Yet the app’s system of secure end-to-end encryption – which means that nobody outside the group can intercept the messages – also attracts those with more sinister intent. The home secretary Amber Rudd suggested earlier this year that it was one of several potential hiding places for those plotting terrorist atrocities – Isis recruiters have been known to use it and Khalid Masood sent a message on the service shortly before killing six people by driving his car into a crowd of pedestrians on Westminster Bridge earlier this year. The FBI, meanwhile, is said to be concerned about its potential use in money laundering, insider trading and other financial crimes.

The biggest danger for ordinary users, however, is that while a group may feel like a safe and private space, it can be anything but. It’s so simple for the distracted to send what was meant to be a private thought around the wrong group, as the Labour MP Lucy Powell found out when she accidentally texted a less than flattering message about frontbench colleagues to the entire women’s parliamentary Labour party.

And, unlike a whispered conversation in real life, WhatsApp leaves an electronic record that can all too easily be leaked by a rogue group member; like human friendships down the ages, it’s only ever as strong as its most gossipy link. Some things, it seems, even technology can’t change.

The Second Source: a group set up by female journalists to tackle sexual harassment in the media, by Rosamund Urwin

Out of touch: ‘Career advice shouldn’t come with a kiss or a grope as part of the deal,’ says Rosamund Urwin.Photograph: Alamy

Sisterhood can emerge in unexpected places. For the past few weeks, WhatsApp has been my haven of female solidarity, my sorority house – a group set up in reaction to the onslaught of tales about sexual harassment is now forging a response to combat this abuse.

Reading her piece was like a punch to the gut. What struck me was that Reynolds – who is younger than me – had had similar (actually far worse) experiences to me when I started as a journalist 10 years ago. It made me think that if nothing changed, this would happen to the 22-year-old of tomorrow, of next year, of the next decade – a stuck record of abuse.

Patterns swiftly emerged. The same names kept coming up – and the same behaviour

Compared with what has since been reported, the harassment I suffered seems mild. There were inappropriate text messages (“Before I die, I will kiss every freckle on your lips” was one) and incessant pestering to go for drinks. Reynolds, in a list of horrors, wrote she had been sent an unsolicited (when does any women actually ask for these?) penis pic before a follow-up introducing her to a contact.

We decided to set up a WhatsApp group for female journalists to talk about their experiences. Five founding members swiftly became 20. We weren’t alone in this: there were other WhatsApp groups set up by women in the media and other industries – most famously a group of political researchers talking about misdemeanours in the House of Commons.

The deal was that no one shared anyone else’s stories. Patterns swiftly emerged. The same names kept coming up – and the same behaviour. Many men invited women for drinks ostensibly to give them career advice, but expecting rather more than a chat about pitching to editors and building a Twitter following.

It became obvious why this type of abuse is so prevalent in the media. It is an informal industry where contacts are all. Freelancers starting out and those doing shifts, especially, have to build a contacts book of editors to ensure they have work. That means asking to meet for a coffee, or perhaps feeling unable to say “no” when the man who can turn off the tap of work for you requests you come for a drink.

There are those who would say these young women are naive in thinking older men would help them out of mere kindness. But that is exactly what usually happens with younger men (although I would note that they, too, can find themselves fleeing cabs to escape roving hands). There is a system of patronage where wisdom gets passed down to the next generation. When the boys ask for it, the advice doesn’t come with the addendum of an invite to a hotel room – a kiss or a grope aren’t part of the deal.

Our WhatsApp group started with stories, but it quickly morphed into a movement. We met up and then launched the Second Source earlier this month, which sets out to tackle harassment in the media – promoting awareness, informing women about their rights and working with organisations to create change. We still organise it all via WhatsApp. The medium isn’t really what matters – we would have done this over email otherwise – but WhatsApp does work particularly well for campaigning. It lends itself to quick decisions and anyone can weigh in with a tweak. It feels egalitarian. The only problem is that in the excitement of a constant flow of messages, it can take over your life. I’ve not paid much attention to the second series of Stranger Things – I’ve been staring at my phone. And being one of the older members (I’m 33), I’m not as competent at it as others – I thought I was using the pointing emoji when I was actually giving everyone the middle finger.

In recent weeks, these “whisper networks” have come under fire, accused of being used to fell men’s careers. That isn’t true, but it reflects the whiff of paranoia that often accompanies women organising. When allegations have emerged, they were from women going public with their experiences – not a whisper but a public shout.

This group has empowered us. When I was sexually harassed, I felt alone, now I realise so many women shared my experiences and we want to use this collective call for change to stop it happening to more women in the future. This revolution will be WhatsApped.

Under siege: Syrian Civil Defence volunteers, known as White Helmets, dig a body out of the rubble after an airstrike in Aleppo.Photograph: Karam Al-Masri/AFP/Getty Images

We use it to contact people in areas completely under siege

The internet in Syria is bad, but WhatsApp doesn’t require a lot of data, so everyone uses it. I’ve been with the Syria Civil Defence, also known as the White Helmets, since 2013 – we are volunteers who go to the scene of attacks to rescue trapped civilians. We have people ready all round Syria. We use WhatsApp groups to organise where more help is needed. It’s especially important for contacting people in areas that are completely under siege, like the countryside around Homs, and areas where there are no longer motorways to travel on. We can hold meetings with these colleagues through WhatsApp.

In April, when there was a large chemical attack in Khan Sheikhoun, the local rescue group used WhatsApp to communicate that they needed more support. Many rescue groups responded and went to the scene where more than 60 people were killed and many more injured. It is difficult to think how this would have been co-ordinated so quickly with so many people in any other way. As a media officer, I used WhatsApp to communicate with people on the ground about what was happening and then put the story on social media to bring more help to the people there. This week, we’ve been using WhatsApp to speak to our team in Ghouta, a Damascus suburb, where there is shelling in the street. WhatsApp makes being in touch during the war easier for us.

Gaysians – Khakan Qureshi: an LGBT group for Asians offering support and organising Pride marches

Streets ahead: ‘We organised for 100 of us to be in the Pride parade this year’

I’m part of a south Asian LGBT support group in Birmingham and someone randomly contacted me online to see if I wanted to help get our community across the UK together for this year’s London Pride – he invited me to a small WhatsApp group to organise it.

It makes each of us stronger. Together we’re making things happen

The group is made up of 19 people around the country, many of them prominent within our community. We named it Gaysians – a portmanteau of gay and Asian. It’s something that we started using between ourselves and it became a byword. It’s 70 years since the partition of India and 50 years since homosexuality was decriminalised in the UK, so it felt like a good time for us to make a statement.

We organised for 100 of us to be in the Pride parade this year. It was a beautiful, hot, sunny day and we really stood out because everyone was in traditional dress – salwar kameez, bright colours, embroidered patterns. It was amazing seeing so many of us pull together. We might have gone to Pride as individuals in the past, but this time we made a stand and said: “We are gay Asians and we’re standing proud.” It felt like our little WhatsApp group made a big difference.

Many of us are trying to reconcile faith and sexual orientation. In our group there are atheists, agnostics, Sikhs, Hindus, Muslims and we discuss the unique challenges we face. There’s one woman who is going through a traumatic time as she prepares to tell her family she is lesbian and we’ve been able to give her assurance and guidance.

I’ve been in lots of these chats that end after a few weeks, but we’ve kept ours going, sending each other articles and support. I love it because it makes us each stronger. We’re like-minded individuals who have come together to make things happen.

People on a night out can be aggressive. We share pictures of them drinking

I set up a group for traders in our area of Cork City to communicate the problems we’re facing. We’ve got 29 different businesses in the group – the Scout shop, McDonald’s, there are public houses, restaurants, craft shops, hairdressers, and shoe shops. This summer there was an increase in antisocial behaviour and it hasn’t been given enough attention by the authorities. People are openly dealing heroin. There’s a lot of drinking happening on the main street, and a lot of aggression. I’m a publican and I try to keep a strict house, but there are off-licences selling countless amounts of alcohol and it’s affecting us all. We use the group to record stuff so that when we go into meetings with City Hall or the garda, we know what we want to talk about. We take pictures of people drinking in the street, urinating, lying on the plinths that are meant for recreation. We’re not being vigilantes. We’ve been told, and we’re very aware, that we can’t use any of this as official evidence against anyone.

We’re all self-employed and don’t really have the time to take off for meetings, so it saves us time. We’ve only used the group in a negative way so far, but we’ve decided we’re going to use WhatsApp to organise a festival in the spring, too.

Love Island – Kem Cetinay and Amber Davies: the two winning couples from this summer’s reality TV hit show share jokes and deal with hurtful tabloid untruths on their group

Kem Cetinay: When we were in the Love Island villa in Mallorca, the four of us were like a little family. Me and Chris [Hughes] were friends, and Amber [Davies, Kem’s girlfriend] and Liv [Attwood, Chris’s girlfriend] were close, too. When we got our phones back after the show, the first thing I did was set up a WhatsApp group for our little clique.

We always share things that are being said about us on social media or in the press, but aren’t true. We wake up every morning and check Twitter and there’s always some new speculation. People will be saying that one of us have split up, and we’ll be messaging each other saying: “Where are they getting this from?”

Our relationship is very public. It’s good to have each other to talk to

There was one time when we were at the airport and Amber said she didn’t want to hold her suitcase so I took it for her. Chris and Liv were standing behind us holding hands, someone took a picture and then used it to say Amber and I were no longer together. I posted it in our group, making a joke that because we weren’t holding hands as Amber was too lazy to hold her suitcase, people are now saying we’re not talking.

We share the memes people make of us. And the tattoos they’ve had. Chris and Liv like to send me and Amber little videos of them when they’re out together, and we’ll send one back.

We all came out of the show in new relationships that are public so it’s good to have each other to talk to about it. Especially me as this is my first relationship.

Chris and I also have our own WhatsApp chat. If I want to talk to anyone about me and my girlfriend, it’s going to be to Chris, and the same with him. We chat about our show and our music. I’m on WhatsApp 24/7. Amber says I’m addicted.

Amber Davies: Kem’s the organiser in general and started the group. He’s always the one who wants to be socialising, so he uses the group to arrange double-dates for the four of us. Liv and I like to use the group to send screenshots of things we find funny that people have Tweeted. Someone Tweeted Liv: “Oh, I wish I could live with Chris and Kem,” and we were messaging between us saying: “We’ll give you 24 hours and see how you get on.”

Chris and Kem’s chat makes me and Liv roll our eyes. They talk about themselves all the time. About the events they’re going to or the filming they’re doing. Liv and I are more like: “What are we wearing on our night out?” We used to respond to the boys more, but now we just ignore them. It’s a typical boyfriend and girlfriend WhatsApp group.

Women’s PLP – Lucy Powell: a group for female MPs

Message mistake: ‘A few days later we could see the funny side,’ says Lucy Powell.Photograph: David Levene for the Guardian

I’m in about eight parliamentary WhatsApp groups. I’m not really desperate to be in any more; with parent groups, family groups and work ones, it can be hard to keep up.

One group I’m in is the Women’s PLP (Parliamentary Labour Party). Diane Abbott and Emily Thornberry are in it, too. At the moment, sexual harassment is causing a lot of traffic. There are different views on what actions we need to follow.

I realised my mistake and spent the next few minutes in absolute horror

Back in January, I had a temporary group with the MPs Alison McGovern and Jess Phillips to draft an article on childcare. When the article was published, there had been a sense offline that our front bench wasn’t that happy. We hadn’t intended it to be confrontational, just a useful intervention in the debate. Later in the day, there was a conversation on the Women’s PLP group about childcare and the article came up. I meant to comment about that conversation in the group of the three of us, but put my message in the Women’s PLP group – about women in that group. [The post read: “We are in the most ludicrous, nonsensical, pretend, unreal, bollocks position as an Opposition… Angela [Rayner] and Tulip [Siddiq] really think they’re going to be Ministers in an actual Labour government very soon.”]

I realised my mistake almost straight away and I spent the next few minutes in absolute horror. The thought of showing my face in Parliament was mortifying.

I texted Angela and Tulip to apologise and posted back in the larger group to apologise to everybody. Obviously, it serves me right, and I’m not expecting any sympathy for it, but I was just a bit cross and thought I was venting to a few mates, not the entire Women’s Parliamentary Labour Party.

There were two hours of tumbleweed on the group. The messages just sitting there and me knowing everybody was logging on at different points and seeing them for the first time. Luckily others realised the best thing to do was to try to move the conversation up the screen and started posting other stuff. I had a sleepless night, but Angela and Tulip were very, very good about it. A few days later we could see the funny side.

It’s made me incredibly careful. We all have times where we say stuff about friends and family, let alone colleagues, to let off steam. But if you commit those things to writing, whether it’s WhatsApp, text or email, then they can stand there for the rest of time without context.